1. Field of the Invention
The present invention relates to non-impact printing apparatus for printing characters, halftone images and the like with small pixels (dots).
2. Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,596,995 dot printers employing recording elements such as LED's (light emitting diodes) and the like are known. Briefly, print apparatus of this type comprise a multiplicity of individually addressable point-like radiation sources that are arranged in a row(s) for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the row(s). Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective image or data bit input signals applied to the driver circuits during an information line period. In forming images upon the photoreceptor, it is highly desirable that the exposure forming each pixel be uniform. In the case of LED's it is known to provide compensation for inherent differences in luminance between the hundreds or thousands of LED's on a print head by, for example, adjusting the current to each LED or by adjusting the duration of exposure. This compensation is known to be provided during the manufacture of the printer apparatus as well as after manufacture when the printer apparatus is in the customer's possession. This latter becomes necessary because over time, the luminance capability of the LED's change due to differential usage. It is, therefore, highly desirable to provide printer apparatus that can automatically maintain a state of uniformity calibration over time for each recording element, especially for apparatus used for pictorial printing.